1. Field of the Invention
The present invention relates to an image forming apparatus. Particularly, the present invention relates to an optical printer for forming an image by an electrophotographic system using an array head and a thermal printer for thermosensible recording using a thermal head.
2. Description of the Related Art
FIG. 1 shows a schematic configuration of a conventional optical printer. Around a photosensitive drum 101 are disposed a charger 102, an LED array head 103, a developer 106, a transfer charger 107, a scraper 109 and an eliminating lamp 110 in this order in the rotating direction of the photosensitive drum 101. The charger 102 charges the surface of the photosensitive drum 101 to a potential of several hundreds volts. A drive circuit 105 drives the LED array head 103 in accordance with an externally input image signal 104. Light emitted by the LED array head 103 forms an electrostatic latent image on the surface of the photosensitive drum 101 previously charged by the charger 102. The electrostatic latent image formed through the exposure by the LED array head 103 is invisible at this point, and is changed into a visible image through the development by the developer 106, that is, the attachment of toner supplied from the developer 106. The transfer charger 107 forms an electric field by using a difference in potential between the surfaces thereof and the photosensitive drum 101. As a result, the toner forming the visible image is transferred onto recording paper conveyed by paper feed rollers 111. The recording paper bearing the toner is allowed to pass through heated rollers in a fixer 108, resulting in fixing the visible toner image onto the recording paper. The scraper 109 scrapes the residual toner that has not attached to the recording paper off the photosensitive drum 101. The entire surface of the photosensitive drum 101 is then exposed by the eliminating lamp 110 so that the electrostatic latent image remained thereon be eliminated before the subsequent image forming operation.
FIG. 2 is a diagram showing a light emitting portion of the conventional LED array head 103. FIG. 3 shows conventional exposure of a plurality of pixels constituting an electrostatic latent image formed on the photosensitive drum 101 (hereinafter referred to as merely the "pixels"). The LED array head 103 includes a substrate 112, a plurality of electrodes 113 and a plurality of light emitting elements 114 both arranged on the substrate 112. The electrodes 113 and the light emitting elements 114 are arranged parallel to the main scanning direction. Each of the light emitting elements 114 has a width W that is sufficiently large for the exposure of the pixels each having a width of W'. A width herein indicates a length in the main scanning direction and a height herein indicates a length in the sub scanning direction.
The height H of each light emitting element 114 is 1/4 of the height H' of each pixel. The light emitting element of this size can expose a quarter of the pixel having the height of H'. Specifically, in the optical printer having the LED array head 103 as shown in FIG. 2, each pixel is divided into four portions in the sub scanning direction and scanning is conducted four times to expose each of the four portions of the pixel, thereby exposing the entire pixel. When the number of the divided portions in a pixel is eight, the height H of the light emitting element is 1/8 of the height H' of the pixel and scanning is conducted eight times to expose the entire pixel. Thus, it is comparatively easy to refine the pixels not along the alignment direction of the light emitting elements 114, that is, in the main scanning direction, but in the rotating direction of the photosensitive drum, that is, in the sub scanning direction.
During the formation of an image, the LED array head 103 shown in FIG. 2 exposes respective pixels 115 as shown in FIG. 3 in accordance with an image signal, a control signal and a pixel clock signal. In this exposure, a quarter of each pixel 115 is independently exposed. In this manner, each pixel is divided into n sub-pixels in the sub scanning direction and scanning is conducted n times. The entire scanning of one pixel is thus completed. During the scanning conducted n times on one pixel, the number of the times of light emission (hereinafter referred to as the number of light emission) by the light emitting element depends upon the density of the pixel according to an externally input image signal. When the number of light emission by the light emitting element is determined to be m (wherein m is an integer; 0.ltoreq.m.ltoreq.n), the sub-pixels up to the mth sub-pixel are successively exposed in the sub scanning direction. For example, when the number of light emission is one, the first sub-pixel alone is exposed. When the number of light emission is two, the first and the second sub-pixels are exposed. An optical printer using the LED array head as shown in FIG. 2 achieves n+1 density levels of gray scale (hereinafter referred to as gray scale levels) in this manner.
Next, a thermal printer using a heat collecting type thermal head will be described referring to FIG. 4, which shows a schematic configuration of a conventional thermal printer. As is shown in FIG. 4, recording paper is fed between a thermal head 120 and a roller 121 in the leftward direction as shown with an arrow in the drawing by convey rollers 116. An ink sheet 119 is wound around and supplied from an ink sheet set roller 118, passes between the recording paper and the thermal head 120 and is rewound around an ink sheet wound roller 117. The thermal head 120 includes a heating element and a drive circuit for the same (neither shown). The thermal head 120 supplies a current to the heating element in accordance with an externally input image signal. The ink on the ink sheet 119 is melted by the heat thus generated by the heating element and attached onto the recording paper. A used portion of the ink sheet 119 having passed between the thermal head 120 and the roller 121 is rewound around the ink sheet rewinding roller 117. Thus, a fresh portion of the ink sheet 119 is always supplied between the thermal head 120 and the roller 121.
In reproducing gray scale levels in such a thermal printer, each pixel is divided into n sub-pixels in the feeding direction of recording paper (i.e., in the leftward direction in FIG. 4) in the same manner as in the optical printer, and the heating element is switched on/off with respect to each sub-pixel. Also in this case, the thermal printer achieves n+1 gray scale levels.